Nitrogen doped A2Nb4O11, process for preparation thereof, and method for degradation of organic pollutants

ABSTRACT

The present invention relates to nitrogen doped A 2 Nb 4 O 11 , which is represented by A 2 Nb 4 O 11-x N x , to a process for the preparation thereof, and to a method for degradation of organic pollutants. The nitrogen doped A 2 Nb 4 O 11  is a new photocatalyst for the photocatalytic degradation of organic pollutants in the waste water. The A 2 Nb 4 O 11-x N x  catalyst may be prepared by substituting some of the O elements of pure A 2 Nb 4 O 11  with N elements, and a process for the preparation thereof comprises a step of doping of nitrogen with a nitrogen source through a solid state reaction. The new nitrogen doped A 2 Nb 4 O 11  catalyst having a general formula A 2 Nb 4 O 11-x N x  has a narrower optical bandgap compared to pure A 2 Nb 4 O 11 , and therefore can be activated under the visible light range and it shows high efficiency in the degradation of organic pollutants.

TECHNICAL FIELD

The present invention relates to the field of photo-catalyst,particularly to nitrogen doped A₂Nb₄O₁₁, process for preparationthereof, and method for degradation of organic pollutants.

STATE OF THE ART

Organic pollutants in water have been causing serious environmentalproblems. Photocatalytic degradation of these pollutants using sunlightis an attractive solution to this global problem. At present, the mostcommonly used photocatalysts are semiconductors, such as ZnO (M. A.Behnajady, N. Modirshahla, R. Hamzavi, J. Hazard. Mater., B 2006, 133226-232; A. Akyol, H. C. Yatmaz, M. Bayramoglu, Appl. Catal. B, 2004,54, 19-24; R. Kavitha, S. Meghani, V. Jayaram, Mater. Sci. Eng. B, 2007,139, 134-140; R. Y. Hong, J. H. Li, L. L. Chen, D. Q. Liu, H. Z. Li, Y.Zheng, J. Ding, Powder Technol., 2009, 189, 426-432; P. Pawinrat, O.Mekasuwandumrong, J. Panpranot, Catal. Commu., 2009, 10, 1380-1385; X.Zhou, Y. Z. Li, T. Peng, W. Xie, X. J. Zhao, Mater. Lett., 2009, 63,1747-1749), TiO₂ (R. A. Damodar, S. J. You, H. H. Chou, J. Hazard.Mater., 2009, 172, 1321-1328; Z. B. Wu, F. Dong, Y. Liu, H. Q. Wang,Catal. Commun., 2009, 11, 82-86; A. Vijayabalan, K. Selvam, R.Velmurugan, M. Swaminathan, J. Hazard. Mater., 2009, 172, 914-921),BiWO₆ (L. W. Zhang, Y. J. Wang, H. Y. Cheng, W. Q. Yao, Y. F. Zhu, Adv,Mater., 2009, 21, 1286+), CdS (H. Y. Zhu, R. Jiang, L. Xiao, Y. H.Chang, Y. J. Guan, X. D. Li., G. M. Zeng, J. Hazard. Mater., 2009, 169,933-940), Fe₂O₃(E. Rodriguez, G. Fernandez, B. Ledesma, P. Alvarez, F.J. Beltran, Appl. Catal. B Envir., 2009, 92, 240-249), and HNbO₃ (X. Li,N. Kikugawa, J. Ye, Chem. Eur. J., 2009, 15, 3538-3545). Thesesemiconductors, however, can only absorb UV light due to their largeband gaps. For better utilization of sunlight and indoor illumination,Nb-based photocatalysts have recently been extensively studied becausethe layered perovskite type niobates can be excited by visible light (X.Li, N. Kikugawa, J. Ye, Chem. Eur. J., 2009, 15, 3538-3545; M. Dion, M.Ganne, M. Tournoux, gMat. Res. Bull, 1981, 16, 1429-1435; J. Yoshimura,Y. Ebina, J. Kondo, K. Domen, A. Tanaka, J. Phys. Chem., 1993, 97,1970-1973; J. H. Wu, Y. H. Cheng, J. M. Lin, Y. F. Huang, M. L. Huang,S. C. Hao, J. Phys. Chem. C, 2007, 111, 3624-3628).

In 1981, Dion reported a family of layered perovskite type niobates,generally formulated as AM₂Nb₃O₁₀ (A=K, Rb, Cs; M=La, Ca, etc.), whichshow noticeable photocatalytic activity (M. Dion, M. Ganne, M. Tournoux,Mat. Res. Bull, 1981, 16, 1429-1435). Soon after that, Yoshimurareported a layered perovskite type niobate, RbPb₂Nb₃O₁₀, that couldphotocatalytically generate H₂ from water by visible light (J.Yoshimura, Y. Ebina, J. Kondo, K. Domen, A. Tanaka, J. Phys. Chem.,1993, 97, 1970-1973). Recently, Wu reported K_(2-x)La₂Ti_(3-x)Nb_(x)O₁₀and their protonated derivatives for water splitting under visible light(J. H. Wu, Y. H. Cheng, J. M. Lin, Y. F. Huang, M. L. Huang, S. C. Hao,J. Phys. Chem. C, 2007, 111, 3624-3628). However, the use of layeredperovskite type niobates as photocatalysts for the degradation oforganic pollutant has received little attention until very recently,when Ye reported efficient photodegradation of Rhodamine B in waterusing nitrogen-doped lamellar niobic acid X. Li, N. Kikugawa, J. Ye,Chem. Eur. J., 2009, 15, 3538-3545). Although these layered perovskitetype niobates and their protonated derivatives show high photoactivityunder visible light irradiation, the structures of these photocatalystsare generally not very stable and are susceptible to collapse even underatmospheric conditions.

We are interested in the use of non-layered niobate salts asphotocatalysts as we anticipate that they would be more robust than thelayered niobates and their protonated derivatives. K₂Nb₄O₁₁ isconstructed from NbO₆ octahedra and has a tetragonal tungsten bronze(TB) crystal structure with triangle, quadrilateral and pentagonaltunnels. The pentagonal and quadrilateral tunnels are occupied by Kcations and the triangle tunnels by Nb cations (M. Lundberg, M.Sundberg, J. Solid State Chem., 1986, 63, 216-230). It has been reportedthat Cu-doping of K₂Nb₄O₁₁ results in increased photocatalytic activityfor the degradation of acid red G under UV irradiation (G. K. Zhang, X.Zou, J. Gong, F. He, H. Zhang, S. Ouvang, H. Liu, J. Molec. Catal. A:Chem., 2006, 255, 109-116).

SUMMARY OF THE INVENTION

The present invention provides a nitrogen doped photocatalyst which isdenoted as A₂Nb₄O₁₁—N, or represented by the following general formula(I)A₂Nb₄O_(11-x)N_(x)  (I)wherein

A is selected from the elements of Group IA of the periodic table; and0<x<1

According to one aspect of the present invention, A in the generalformula (I) is Li, Na, K, Rb or Cs. Most preferably, A is K.

According to one aspect of the present invention, the compound of thegeneral formula (I) of the present invention has a tetragonal tungstenbronze crystal structure.

According to one aspect of the present invention, the compound of thegeneral formula (I) of the present invention may be used as aphotocatalyst. Preferably, said photocatalyst can be activated undervisible lights.

The present invention further provides a process for the preparation ofthe compound of the general formula (I) of the present invention,comprising the steps of:

1) surface acidification of A₂Nb₄O₁₁, wherein A₂Nb₄O₁₁ is immerged in anacidic solution, filtered, washed and dried; and

2) nitrogen doping of A₂Nb₄O₁₁ to obtain A₂Nb₄O_(11-x)N_(x), whereinA₂Nb₄O₁₁ obtained in step 1) is mixed with a nitrogen source and heated,the product is washed to remove residue nitrogen source adsorbed on thesurface of the product and dried.

The A₂Nb₄O_(11-x)N_(x) catalyst of the present invention is prepared byreplacing some of the O elements in pure A₂Nb₄O₁₁ with N elements, and aprocess for the preparation thereof comprises a step of doping ofnitrogen with a nitrogen source through a solid state reaction.

According to one aspect of the present invention, the nitrogen source inthe above process may be an ammonium salt or a nitrogen-containingorganic compound, such as ammonium carbonate or urea.

According to one aspect of the present invention, the acidic solutionused in step 1) of the above process may be selected from the groupconsisting of hydrochloric acid, nitric acid, sulfuric acid, orphosphoric acid. Preferably, the acidic solution has a concentration of1-10 mol/L.

According to one aspect of the present invention, in step 1), the ratioof the weight of A₂Nb₄O₁₁ to the volume of the acidic solution may befrom 1 g:10 ml to 1 g:600 ml.

According to one aspect of the present invention, in step 1), theduration of the immersing may be 10-96 hours.

According to one aspect of the present invention, in step 1), thewashing may be performed with distilled water; the drying may beperformed under a temperature of 20-300° C., and the duration of thedrying may be above 10 hours.

According to one aspect of the present invention, in step 2), the weightratio of A₂Nb₄O₁₁ and the nitrogen source may be from 1:0.5 to 1:1.0.

According to one aspect of the present invention, in step 2), theheating may be performed under a temperature of 300-600° C.

According to one aspect of the present invention, in step 2), theduration of the heating may be 1-10 hours.

According to one aspect of the present invention, in step 2), theproduct may be washed with acetone and/or distilled water to removeresidue nitrogen source, such as alkaline species, adsorbed on thesurface of the product.

According to one aspect of the present invention, in step 2), the dryingmay be performed under a temperature of 20-300° C., and the duration ofthe drying may be 10-96 hours.

A₂Nb₄O₁₁ used in step 1) of the present invention has a tetragonaltungsten bronze crystal structure. It may be obtained commercially, ormay be prepared according to a process known in the art, or may beprepared according a process wherein A₂Nb₄O₁₁ is prepared by heating amixture of Nb₂O₅ and A₂CO₃ for several hours. In said process, theheating may be performed under a temperature of 800-1200° C.; theduration of the heating may be 8-50 hours; and the ratio of Nb₂O₅ andA₂CO₃ may be from 3:1 to 1:10.

The present invention further provides a method for degradation oforganic pollutants, comprising contacting the organic pollutants withthe compound of the general formula (I) of the present invention. Asused herein, the term “organic pollutants” generally refers to organicsubstances which may cause adverse effects to human health and theenvironment. Preferably, the organic pollutants are those difficult todecompose in waste water. As used herein, the organic pollutantsdifficult to decompose are organic compounds which may be present inwaste water for a long time without decomposition under ambientconditions, such as Orange G (OG) and bisphenol A (BPA).

It has been proven that the new catalyst having a general formulaA₂Nb₄O_(11-x)N_(x) has a narrower optical bandgap compared to pureA₂Nb₄O₁₁, and therefore can be activated under the visible light rangeand it shows high efficiency in the degradation of organic pollutants,especially organic pollutants difficult to decompose. In addition, theprocess for the synthesis of the nitrogen doped A₂Nb₄O₁₁ is simple andcan be performed on a large scale, and the process for nitrogen dopingis less expensive than conventional sputtering ones. The presentphotocatalyst has the advantages of non-toxicity, chemical inertness,high stability under light irradiation, and high photo efficiency undervisible light, and is therefore a superior photocatalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee. The present disclosure will become more fullyunderstood from the detailed description given herein below forillustration only, and thus are not limitative of the presentdisclosure, and wherein:

FIG. 1. Molecular structures of OG and BPA and their UV-Vis absorptionspectrum.

FIG. 2. XRD patterns of: A) K₂Nb₄O₁₁ B) K₂Nb₄O₁₁—N and C) standardK₂Nb₄O₁₁ (JCPDS 31-1059).

FIG. 3. SEM images of: A) K₂Nb₄O₁₁ and B) K₂Nb₄O₁₁—N.

FIG. 4. XPS spectra in the whole energy range. A) K₂Nb₄O₁₁ and B)K₂Nb₄O₁₁—N.

FIG. 5. XPS spectra of the elements in K₂Nb₄O₁₁ (A) and K₂Nb₄O₁₁—N (B).1, 2 and 3 refer to Nb, O and N, respectively.

FIG. 6. A) UV-vis diffuse reflectance spectra of K₂Nb₄O₁₁ andK₂Nb₄O₁₁—N; B) The curve deduced from A according to the equation(ahv)²=A(hv−E_(g)).

FIG. 7. Photoluminescence (Ph) emission spectra of (a) K₂Nb₄O₁₁ and (b)K₂Nb₄O₁₁—N.

FIG. 8. Plot of C/C_(o) (C is the concentration at time t, C_(o) is theinitial concentration) versus time for the photo-degradation of OG using330 nm cut off filter with a) K₂Nb₄O₁₁, b) K₂Nb₄O₁₁—N and c) TiO₂ P25.

FIG. 9. Plot of C/C_(o) (C is the concentration at time t, C_(o) is theinitial concentration) versus time for photo-degradation of OG using 399nm cut off filter with a) K₂Nb₄O₁₁, b) TiO₂ P25 and c) K₂Nb₄O₁₁—N.

FIG. 10. Spectral changes of OG during irradiation using 399 nm cutofffilter with K₂Nb₄O₁₁—N.

FIG. 11. Plot of (TOC)/(TOC)_(o) [(TOC) is the total organic carbon attime t, (TOC)_(o) is the initial total organic carbon] versus time forphoto-degradation of OG using a 399 nm cut off filter with K₂Nb₄O₁₁—N.

FIG. 12. Plot of C/C_(o) (C is the concentration at time t, C_(o) is theinitial concentration) versus time for photo-degradation of OG using a399 nm cut off filter with a) Aged K₂Nb₄O₁₁—N; b) Fresh K₂Nb₄O₁₁—N.

FIG. 13. Plot of C/C_(o) (C is the concentration at time t, C_(o) is theinitial concentration) versus time for photo-degradation of BPA using a399 nm cutoff filter with a) no catalyst; b) Nb₂O₅; c) K₂Nb₄O₁₁; d)Degussa TiO₂ P25; e) K₂Nb₄O₁₁—N.

FIG. 14. Scheme of the band structure of K₂Nb₄O₁₁—N and visiblephotocatalytic processes.

FIG. 15. Plot of C/C_(o) (C is the concentration at time t, C_(o) is theinitial concentration) versus time for photo-degradation of BPA byK₂Nb₄O₁₁—N at different pH; a) pH3; b) pH6; c) pH10.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides more effective photocatalysis withsunlight by nitrogen doped A₂Nb₄O₁₁. Specifically, the efficiency ofK₂Nb₄O₁₁—N for the photodegradation of organic pollutants under visiblelight (>399 nm) irradiation has been investigated by using Orange G (OG)and bisphenol A (BPA) as substrates. Orange G (OG), a synthetic azo dyewith formula C₁₆H₁₀N₂Na₂O₇S₂(7-hydroxy-8-phenylyazo-1,3-naphthalenedisulfonic acid disodium salt),is an endocrine disruptor (B. Muktha, G. Madras, T. N. G. Row, U.Scherf, S. Patil, Conjugated polymers for photocatalysis J. Phys. Chem.B, 2007, 111, 7994-7998; P. Mahata, G. Madras, S. Natarajan, Catal.Lett., 2007, 115, 27-32; P. Mahata, G. Madras, S, Natarajan, J. Phys.Chem. B, 2006, 110, 13759-13768; S. Mahapatra, G. Madras, T. N. GuruRow, J. Phys. Chem. C, 2007, 111, 6505-6511; M. Sokmen, A. O zkan, J.Photochem. Photobiol. A—Chem., 2002, 147, 77-81; C. Guillard, H.Lachheb, A. Houas, M. Ksibi, E. Elaloui, J.-M. Herrmann, J. Photochem.Photobiol. A—Chem., 2003, 158, 27-36; J. Sun, X. Wang, J. Sun, R. Sun,S. Sun, L. Qiao, J. Mol. Catal. A—Chem., 2006, 260, 241-246; J. M. Kwon,Y. H. Kim, B. K. Song, S. H. Yeom, B. S. Kim, J. B. Im, J. Hazard.Mater., 2006, 134, 230-236; G. Sivalingam, K. Nagaveni, M. S. Hegde, G.Madras, Appl. Catal. B—Environ., 2003, 45, 23-38; C. Hachem, F.Bocquillon, O. Zahraa, M. Bouchy, Dyes Pigment., 2001, 49, 117-125; S.Yang, L. Lou, K. Wang, Y. Chen, Appl. Catal. A—Gen., 2006, 301, 152-157;K. Nagaveni, G. Sivalingam, M. S. Hegde, G. Madras, Appl. Catal.B—Environ., 2004, 48, 83-93; J. H. Sun, L. P. Qiao, S. P. Sun, G. L.Wang, Journal of Hazard. Mater., 2008, 155, 312-319). It is a commonreagent in molecular biology and is used in histology as a stainingagent. Bisphenol A is also a known endocrine disruptor which has beenwidely used for the production of polycarbonate (PC) and epoxy resinsused in food containers R. Tsekov, E. Evstatieva, K. W. Stockelhuber, P.G. Smirniotis, Progr Colloid Polym Sci., 2004, 126, 117-120; D. Beydoun,R. Amal, G. K.-C. Low, S. McEvoy, J. Phys. Chem. B, 2000, 104,4387-4396; G. K. Zhang, X. Zou, J. Gong, F. He, H. Zhang, S. Ouyang, H.Liu, J. Molec. Catal. A: Chem., 2006, 255, 109-116; B. Muktha, G.Madras, T. N. G. Row, U. Scherf, S. Patil, Conjugated polymers forphotocatalysis, J. Phys. Chem. B, 2007, 111, 7994-7998; P. Mahata, G.Madras, S, Natarajan, Catal. Lett., 2007, 115, 27-32). Its concentrationin wastewaters is increasing, which has become a worldwide pollutionproblem. The molecular structures and the UV-Vis absorption spectra ofOG and BPA are shown in FIG. 1. OG absorbs strongly in the visibleregion but BPA absorbs only in the UV region. The results of thephotodegradation experiments of the present invention show that theK₂Nb₄O₁₁—N photocatalyst has a much higher visible light activity thanpure K₂Nb₄O₁₁ or Nb₂O₅, and it is also more active than Degussa P25TiO₂. These results indicate that the photoactivity of K₂Nb₄O₁₁ has beenprofoundly increased by nitrogen doping.

In order to prepare the nitrogen doped photocatalyst having the generalformula A₂Nb₄O_(11-x)N_(x) to fulfill the object of the presentinvention, the process for the preparation thereof are exemplarilydescribed with examples.

As the present invention may be embodied in several forms withoutdeparting from the spirit thereof, it should be understood that theembodiments of the present invention are not limited by any of thedetails of the description. Unless otherwise specified, it should beconstrued that all changes and modification of the embodiments of thepresent invention are within the scope as defined in the appendedclaims. Meanwhile, all the references cited in the present applicationare incorporated herein by reference in their entirety.

PREPARATION EXAMPLES Example 1

a) K₂Nb₄O₁₁ was prepared by heating a mixture of 0.2 g Nb₂O₅ and 0.8 gK₂CO₃ at 900° C. for 24 hours;

b) 1.0 g K₂Nb₄O₁₁ was immersed in 60 mL of 5 mol/L nitric acid solutionfor 48 hours, and then the product was filtered, washed with distilledwater, and dried at 100° C. for 20 hours;

c) 10.0 g urea and 1.0 g K₂Nb₄O₁₁ were mixed and finely milled, andheated at 600° C. for 3 hours to provide a yellow product, which waswashed with acetone to remove any residual alkaline species adsorbed onthe surface of the product, and dried at 100° C. for 24 hours.

Example 2

a) K₂Nb₄O₁₁ was prepared by heating a mixture of 0.2 g K₂CO₃ and 0.8 gNb₂O₅ at 900° C. for 24 hours;

b) 1.0 g K₂Nb₄O₁₁ was immersed in 60 mL of 5 mol/L nitric acid solutionfor 48 hours, and then the product was filtered, washed with distilledwater, and dried at 100° C. for 20 hours;

c) 10.0 g K₂Nb₄O₁₁ and 2.0 g urea were finely milled, and heated at 400°C. for 4 hours. The resulted yellow product was washed with acetone anddistilled water, and dried at 70° C. for 24 hours.

Example 3

a) K₂Nb₄O₁₁ was prepared by heating a mixture of 0.2 g K₂CO₃ and 0.8 gNb₂O₅ at 1100° C. for 10 hours;

b) 1.0 g K₂Nb₄O₁₁ was emerged immersed in 300 mL of 3 mol/L hydrochloricacid solution for 48 hours, and then the product was filtered, washedwith distilled water, and dried at 100° C. for 20 hours;

c) 1.0 g K₂Nb₄O₁₁ and 10.0 g ammonium bicarbonate were finely milled,and heated at 400° C. for 4 hours. The resulted yellow product waswashed with acetone and distilled water, and dried, at 70° C. for 24hours.

Example 4

a) Na₂Nb₄O₁₁ was prepared by heating a mixture of 0.2 g Na₂CO₃ and 0.8 gNb₂O₅ at 900° C. for 24 hours;

b) 1.0 g Na₂Nb₄O₁₁ was immersed in 3060 mL of 3 mol/L nitric acidsolution for 96 hours, and then the product was filtered, washed withdistilled water, and dried at 300° C. for 10 hours;

c) 1.0 g Na₂Nb₄O₁₁ and 0.5 g urea were finely milled, and heated at 400°C. for 24 hours. The resulted yellow product was washed with acetone,and dried at 100° C. for 24 hours.

Example 5

a) Na₂Nb₄O₁₁ was prepared by heating a mixture of 0.2 g Na₂CO₃ and 0.8 gNb₂O₅ at 1100° C. for 24 hours;

b) 1.0 g Na₂Nb₄O₁₁ was immersed in 60 mL of 5 mol/L nitric acid solutionfor 48 hours, and then the product was filtered, washed with distilledwater, and dried at 100° C. for 24 hours;

c) 1.0 g Na₂Nb₄O₁₁ and 10.0 g urea were finely milled, and heated at400° C., for 6 hours. The resulted yellow product washed with acetone,and dried at 100° C. for 24 hours.

Example 6

a) Na₂Nb₄O₁₁ was prepared by heating a mixture of 0.2 g Nb₂O₅ and 0.8 gNa₂CO₃ at 900° C. for 24 hours;

b) 1.0 g Na₂Nb₄O_(ii) was immersed in 60 mL of 5 mol/L nitric acidsolution for 48 hours, and then the product was filtered, washed withdistilled water, and dried at 100° C. for 20 hours;

c) 10.0 g urea and 1.0 g Na₂Nb₄O₁₁ were mixed and finely milled, andheated at 400° C. for 24 hours to provide a yellow product, which waswashed with acetone to remove any residues adsorbed on the surface ofthe product, and dried at 100° C. for 24 hours.

TEST EXAMPLES

The nitrogen doped. K₂Nb₄O₁₁ used in the test Examples is K₂Nb₄O₁₁—Nprepared in Example 2.

1. Characterization of the Compound Having the General Formula (I)

The instruments for characterization include: powder X-ray diffraction(XRD), scanning electron microscopy (SEM), X-ray photoelectronspectroscopy (XPS), UV/Vis diffuse reflectance and photoluminescencespectroscopy (PL). The XRD analysis was performed on a Rigaku D-maxX-ray diffractometer with Cu K_(α) irradiation (λ=1.5406 Å) at ascanning speed of 0.025°/sec over the scanning range of 20-70°. Themorphologies were examined by a Philips XL30 environmental scanningelectron microscope (ESEM) at an accelerating voltage of 10 kV. Thesurface analysis was done with a Leybold Heraeus-Shengyang SKL-12electron spectrometer equipped with a VG CLAM 4 MCD electron energyanalyzer, with Al—Kα as the excitation source. UV-Vis diffusereflectance was performed on a Perkin Elmer Lambda 750 UV-VisSpectrophotometer. Photoluminescence (PL) spectra were measured using aFluoroMax-3 spectrofluorimeter equipped with a pulsed xenon lamp aslight source.

2. Photo-Catalytic Degradation Measurements

A 200 W xenon arc lamp (Newport, Model 71232) was used as the lightsource. OG or BPA aqueous solution (30 ml, 20 mg/L) and thephotocatalyst (˜10 mg, nitrogen doped K₂Nb₄O₁₁ prepared in Example 2)were placed into a quartz tube reactor (12 mm in diameter and 200 mm inlength) and the mixture was sonicated for 5 minutes to disperse thecatalyst in the OG or BPA aqueous solution. The distance between theliquid surface and the light source was about 11 cm. Before thephotoirradiation, the mixture was stirred in the dark for one hour so asto establish adsorption-desorption equilibrium on the surface of thecatalyst for OG or BPA. The Infrared and UV light emitted from theXe-lamp was filtered by a water jacket and a cutoff filter (Scott AG KV399). Samples were collected at regular time intervals and centrifugedbefore Analysis. The concentrations of OG or BPA were measured with aShimadzu UV-1700 UV-Vis spectro-photometer, wherein the OG or BPAconcentration is proportional to its absorbance.

3. Results and Discussion 3.1 XRD

Powder X-ray diffraction (XRD) shows nearly identical patterns forK₂Nb₄O₁₁ and K₂Nb₄O₁₁—N prepared in Example 2. Typical XRD patterns ofK₂Nb₄O₁₁ and K₂Nb₄O₁₁—N are shown in FIGS. 2A and 2B, respectively. ForK₂Nb₄O₁₁ (FIG. 2A), all the diffraction peaks can be indexed as atetragonal tungsten bronze structure (JCPDS 31-1059) with latticeconstants of a=0.126 nm and c=0.398 nm. The XRD pattern of nitrogendoped K₂Nb₄O₁₁ nearly identical to that of undoped sample, as shown inFIG. 2B, indicating that there is no effect of nitrogen doping on thecrystal structure of K₂Nb₄O₁₁, which suggests that (loping occurs onlyon the surface.

3.2 SEM

The SEM photographs of K₂Nb₄O₁₁ and K₂Nb₄O₁₁—N are shown in FIGS. 3A and3B, respectively. FIG. 3A shows that the sizes and shapes of particlesare inhomogeneous and the surface is clean. However, when K₂Nb₄O₁₁ washeated together with urea at 400° C., the surface of the sample becameflock-like, but the sizes and shapes of the particles were notsignificantly changed. The SEM and XRD results together indicate thatnitrogen doping does not affect the morphology and crystal structure ofK₂Nb₄O₁₁, but affects the surface profile of the sample, as nitrogendoping occurs mainly on the surface of the sample,

3.3 XPS

The X-ray photoelectron spectroscopy (XPS) analysis is an importantmethod to determine the composition and the chemical state of theelements. The XPS spectra of K₂Nb₄O₁₁ and K₂Nb₄O₁₁—N in wide energyrange are shown in FIG. 4. No significant contamination, besides carbon,is found in the spectra. The binding energy was determined by referenceto C 1s line at 284.8 eV. In the whole energy range spectrum shown inFIG. 4, the elements K, Nb and O can be observed in K₂Nb₄O₁₁ andK₂Nb₄O₁₁—N. However, N can only be seen in K₂Nb₄O₁₁—N, indicatingsuccessful doping of N onto the surface of K₂Nb₄O₁₁. The N concentrationis calculated to be 3.9 atom % using the equation,

$C_{N} = \frac{\frac{I_{N}}{S_{N}}}{\sum\limits_{i}\frac{I_{i}}{S_{i}}}$

Where C_(N) is the nitrogen concentration, I_(N) and I_(i) are the peakintensities of nitrogen and other elements, respectively; S_(N) andS_(i) are the relative sensitivity factors of nitrogen and otherelements, respectively.

To further determine the chemical states of the elements Nb, O and N,core level XPS spectra of K₂Nb₄O₁₁ and K₂Nb₄O₁₁—N are shown in FIG. 5.FIG. 5-A1 and B1 show that the binding energies of Nb 3d_(5/2) forK₂Nb₄O₁₁ and K₂Nb₄O₁₁—N are 206.8 and 206.9 eV, respectively, which areconsistent with the reported values (G. K. Zhang, Y. J. Hu, X. M. Ding,J. Zhou, J. W. Xie, J. Solid State Chem., 2008, 181, 2133-2138). Thechemical shifts of the binding energies of Nb 3d_(5/2) in these twomaterials are small. However, the full widths at half maximum (FWHM) ofthe Nb 3d_(5/2) peaks are different. The FWHM of Nb 3d_(5/2) forK₂Nb₄O₁₁ and K₂Nb₄O₁₁—N is 1.7 and 1.9 eV, respectively. The broadeningof the Nb 3d_(5/2) peak indicates that the electron density on the Nbatoms in K₂Nb₄O₁₁—N is higher than that in K₂Nb₄O₁₁ (T. Shishido, M.Oku, S. Okada, K. Kudou, J. Ye, T. Sasaki, Y. Watanabe, N. Toyota, H.Horiuchi, T. Fukuda, J. Alloy. Comp., 1998, 281, 196-201). The main O 1speak at 530.8 and 530.6 eV in FIGS. 5 A2 and B2 are assigned to latticeoxygen of K₂Nb₄O₁₁ and K₂Nb₄O₁₁—N, respectively (A. Molak, E. Talik, M.Kruczek, M. Paluch, A. Ratuszna, Z. Ujma, Mater. Sci. and Engo. B, 2006,128, 16-24; S.-Y. Lai, Y. Qiu, S. Wang, J. Catal., 2006, 237, 303-313).A higher binding energy shoulder is found for both samples at about532.6 eV, this is assigned to a mixture of surface hydroxyl andcarbonate groups. Nitrogen is found only on K₂Nb₄O₁₁—N and thecore-level N1s XPS is shown in FIG. 5-B3. The N1s spectrum is dividedinto two components with peak energies of 398.7 and 400.8 eV,respectively, which are assigned to be (N)_(i)/(NO)_(O) and(NO)_(i)/(NO₂)_(O), respectively R. Asahi, T. Morikawa, Chem. Phys.,2007, 339, 57-63). (N)_(i) represents N in the interstitial space,(NO)_(O) denotes NO sitting at the site for the lattice O. Similarly,(NO)_(i) and (NO₂)_(O) designate the interstitial NO and substitutionalNO₂ for the lattice 0, respectively.

3.4 UV-Vis Diffuse Reflectance

The light absorption of the samples can be measured with UV/Vis diffusereflectance spectroscopy (M. A. Butler, J. Appl. Phys., 1997, 48,1914-1920). FIG. 6 shows the UV/Vis diffuse reflectance spectra ofK₂Nb₄O₁₁ and K₂Nb₄O₁₁—N. It is known that the optical absorptioncoefficient near the band edge follows the equation (ahv)²=A(hv−E_(g)),wherein a, h, v, E_(g) and A are the absorption coefficient, Planckconstant, light frequency, band gap, and a constant, respectively. Fromthis equation, the band gaps can be calculated to be 3.27 eV and 3.12 eVfor K₂Nb₄O₁₁ and K₂Nb₄O₁₁—N, respectively. The red-shift of theabsorption wavelength of K₂Nb₄O₁₁—N compared with that of K₂Nb₄O₁₁,indicating that nitrogen-doping has a narrowing effect on the band gapof the material.

3.5 Photoluminescence (PL)

Photoluminescence emission spectra of semiconductors are related to thetransport/relaxation behavior of the photo-induced electrons and holes,and thus can be used to determine band gaps, and to detect impuritiesand defects (Y. C. Zhu, C. X. Ding, J. Solid State Chem., 1999, 145,711-715). In order to study the effect of nitrogen doping on the bandgap of K₂Nb₄O₁₁, PL spectra are shown in FIG. 7. There is a peak atabout 370 nm for both K₂Nb₄O₁₁ and K₂Nb₄O₁₁—N, which is due to the bandgap of K₂Nb₄O₁₁ crystals. On the other hand, K₂Nb₄O₁₁—N has anadditional broad emission peak from 380 to 600 nm (C. Yu, J. C. Yu,Catal. Lett., 2009, 129, 462-470; Y. Qiu, S. Yang, Advanced FunctionalMaterials, 2007, 17, 1345-1352; J. C. Yu, J. G. Yu, W. K. Ho, Z. T.Jiang, L. Z. Zhang, Chem. Mater., 2002, 14, 3808-3816), which confirmsthe effect of nitrogen doping on narrowing the band gap of K₂Nb₄O₁₁—N.

3.6 Photocatalytic Degradation of Orange G by UV and Visible Light

The results of the photo-degradation of OG using K₂Nb₄O₁₁ and K₂Nb₄O₁₁—Nas photocatalysts are shown in FIG. 8. The degradation of OG wasnegligible after 4 h when K₂Nb₄O₁₁ was used as the photocatalyst. On theother hand, when K₂Nb₄O₁₁—N was used, nearly 90% OG was degraded after 2h of irradiation, indicating that nitrogen doping greatly enhances thephotocatalytic activity of the K₂Nb₄O₁₁. Control experiments show thatboth light and the photocatalyst are required for the degradation of OG.The degradation of OG by K₂Nb₄O₁₁—N is only slightly less efficient thanTiO₂ P25 when 330 nm cutoff filter is used. On the other hand, when 399nm cutoff filter is used, the photocatalytic activity of K₂Nb₄O₁₁—N ismuch higher than that of TiO₂ P25. As shown in FIG. 9, nearly 90% of OGis degraded over the K₂Nb₄O₁₁—N after 12 h of photoirradiation when a399 nm cutoff filter is used, while only 46% is degraded over TiO₂ P25.

FIG. 10 shows the spectral changes of OG during irradiation using 399 nmcutoff filter with K₂Nb₄O₁₁—N. The main absorption band of OG is ataround 478 nm, which decreases with time upon irradiation, but theλ_(max) does not change, indicating that the photodegradation does notoccur by a dye self-photosensitized oxidative mechanism (T. Wu, G. Liu,J. Zhao, H. Hidaka, N. Serpone, J. Phys. Chem. B, 1998, 102, 5845-5851).Apart from the peak at 478 nm, the photodegradation of OG by K₂Nb₄O₁₁—Nalso results in the disappearance of the peak at 330 nm, indicating thatboth the OG chromophores and the aromatic rings have been destroyed (X.Li, N. Kikugawa, J. Ye, Chem. Eur. J., 2009, 15, 3538-3545). Also thetotal organic carbon (TOC) value of the solution decreases byapproximately 25% at 90% OG conversion after 12 h of irradiation (seeFIG. 11), indicating that OG is mainly degraded to aliphatic organiccompounds and is only partially mineralized to CO₂ and/or CO.

To assess the stability of the photocatalyst, a sample of K₂Nb₄O₁₁—N wasaged under ambient conditions for six months and its photocatalyticactivity was then tested. As shown in FIG. 12, the photocatalyticactivity of the aged K₂Nb₄O₁₁—N sample decreases by only about 6%compared with the freshly prepared sample, indicating that K₂Nb₄O₁₁—Nphotocatalyst is reasonably stable when stored under ambient conditions.

3.7 Photocatalytic Degradation of BPA by Visible Light

It has been suggested by several authors that the photocatalyticdegradation of dyes under visible light may be induced byself-photosensitization of the dye rather than by the catalyst (M. R.Hoffmann, S. T. Martin, W. Y. Choi, D. W. Bahnemann, Chem. Rev., 1995,95, 69-96; A. Mills, J. S. Wang, J. Photochem. Photobiol. A., 1999, 127,123-134; M. Mrowetz, W. Balcerski, A. J. Colussi, M. R. Hoffmann, J.Phys. Chem. B., 2004, 108, 17269-17273; X. L. Yan, T. Ohno, K.Nishijima, R. Abe, B. Ohtani, Chem. Phys. Lett., 2006, 429, 606-610; B.Ohtani, Chem. Lett., 2008, 37, 217-229; G. S. Wu, T. Nishikawa, B.Ohtani, A. C. Chen, Chem. Mater., 2007, 19, 4530-4537; R. Abe, H.Takami, N. Murakami, B. Ohtani, J. Am. Chem. Soc., 2008, 130, 7780-7781;F. Amano, O.-O. Prieto-Mahaney, Y. Terada, T. Yasumoto, T. Shibayama, B.Ohtani, Chem. Mater., 2009, 21, 2601-2603; F. Amano, A. Yamakata, K.Nogami, M. Osawa, B. Ohtani, J. Am. Chem. Soc., 2008, 130, 17650-17651).In order to understand the photoactivity of K₂Nb₄O₁₁—N under visiblelight, Bisphenol A (BPA), which is a colorless pollutant, was selectedas another probe substrate. For comparison, Nb₂O₅, pure K₂Nb₄O₁₁ andDegussa TiO₂ P25 were also used as photocatalysts and the results areshown in FIG. 13.

In the absence of a photocatalyst, the concentration of BPA remainedvirtually unchanged even after 20 h of visible light irradiation (399 nmcutoff). Also, BPA was not degraded by the photocatalysts in the dark.However, upon visible light irradiation in the presence of K₂Nb₄O₁₁—N,90% of BPA was degraded after 6 h. This photoactivity is higher thanthat of Degussa TiO₂ P25, and is much higher than that of pure K₂Nb₄O₁₁and Nb₂O₅. These results confirm that nitrogen doping greatly enhancesthe photoactivity of K₂Nb₄O₁₁.

3.8 Mechanism for the Photocatalytic Activity of K₂Nb₄O₁₁—N

Taking K₂Nb₄O₁₁—N as an example, the band structure for the K₂Nb₄O₁₁—Nis proposed, as schematically shown in FIG. 14. In the K₂Nb₄O₁₁—Nphotocatalyst, there exist isolated N 2p states above the valence-bandmaximum of K₂Nb₄O₁₁, which give rise to the strong absorptionenhancement in the visible region. Under visible light irradiation,electron and hole pairs would be generated between impurity N 2p statesand the conduction band of Nb 4d (equation 1).K₂Nb₄O₁₁—N+visible light

h ⁺ +e ⁻ _(CB)  (1)

The excited electrons e⁻ _(cB) in the conduction band would move to thesurface and combine with surface-adsorbed oxygen to produce O₂ ^(.−)superoxide anion radicals. The O₂ ^(.−) radicals could then react withH₂O to produce .OH radicals (equation 2), which are known to be one ofthe most oxidizing species. On the other hand, the reactive holes h′would react with adsorbed OH⁻ on the catalyst surface to also form .OHradicals (equations 3-4).O_(2ads) ^(.−)+2H₂O→.OH⁻+H₂O₂  (2)H₂O

OH⁻ _(ads)+H⁺  (3)OH_(ads) ⁻ +h ⁺→.OH  (4)

The .OH radicals would react with OG and BPA to produce H₂O and CO₂ viavarious intermediates.

In order to provide more evidence to support the proposed mechanism, theeffects of pH on the photocatalytic degradation of BPA by K₂Nb₄O₁₁—Nwere also studied (see FIG. 15). It was found that photocatalyticactivity of K₂Nb₄O₁₁—N increases when the solution pH decreases. Thismay be explained by the processes shown in equations 5-7, which occur inthe presence of H⁺.O_(2ads) ^(.−)+H⁺→HO_(2ads) ^(.)  (5)HO_(2ads) ^(.)+H₂O→.OH_(ads)+H₂O_(2ads)  (6)H₂O_(2ads) +e _(CB) ⁻→.OH_(ads)+OH_(ads) ⁻  (7)These processes facilitate trapping of the electrons in the conductionband of K₂Nb₄O₁₁—N which produces .OH_(ads). This trapping mechanismretards the recombination of electron-hole pairs and allows a moreefficient charge separation. Hence, the transfer of trapped electrons todissolved oxygen in the solution would be enhanced and more holes andhydroxyl radicals would be available for the oxidation of BPA on thecatalyst surface as well as in the solution phase. This pH effectsupports our proposed mechanism for the photocatalytic activity ofK₂Nb₄O₁₁—N under visible light.

4. Conclusions

In the present invention, A₂Nb₄O₁₁—N has been prepared, fullycharacterized and used for the photodegradation of OG and BPA. XRD andSEM show that the crystal structures of K₂Nb₄O₁₁—N and K₂Nb₄O₁₁ arenearly identical, but the surface profile has been changed significantlydue to the nitrogen doping. XPS and PL indicate that the nitrogen dopingprimarily occurs at the surface of K₂Nb₄O₁₁, while UV/Vis diffusereflectance data further reveal that nitrogen doping narrows the bandgap of K₂Nb₄O₁₁. The photocatalytic activity of the K₂Nb₄O₁₁—N has beenevaluated by photodegradation of OG and BPA under visible lightirradiation. The results show that the photocatalytic activity ofK₂Nb₄O₁₁—N is significantly higher than that of pure K₂Nb₄O₁₁ andDegussa TiO₂ P25 under visible light irradiation, highlighting theimportance of nitrogen doping of K₂Nb₄O₁₁. Overall, we have for thefirst time prepared and characterized A₂Nb₄O₁₁—N with highphotocatalytic activity even with visible light illumination. Moreover,this photocatalyst is very stable (at least six months under ambientconditions), its preparation is simple and highly reproducible, and itis easy to separate from the solution by simple centrifugation.

What is claimed is:
 1. A compound, which is a nitrogen doped A₂Nb₄O₁₁represented by the following general formula (I)A₂Nb₄O_(11-x)N_(x)  (I) wherein A is selected from the elements of GroupIA of the periodic table; and 0<x<1.
 2. The compound of claim 1, whereinA is Li, Na, K, Rb or Cs.
 3. The compound of claim 1, wherein thecompound has a tetragonal tungsten bronze crystal structure.
 4. Thecompound of claim 1, wherein the compound is a photocatalyst.
 5. Thecompound of claim 4, wherein the photocatalyst is activated undervisible lights.
 6. A process for the preparation of the compound ofclaim 1, comprising the steps of: 1) surface acidification of A₂Nb₄O_(i)1, wherein A₂Nb₄O₁₁ is immersed in an acidic solution, filtered, washedand dried; and 2) nitrogen doping of A₂Nb₄O₁₁ to obtainA₂Nb₄O_(11-x)N_(x), wherein A₂Nb₄O₁₁ obtained in step 1) is mixed with anitrogen source and heated, the heated mixture is washed to removenitrogen source residue adsorbed on the surface of nitrogen dopedA₂Nb₄O₁₁ and dried.
 7. The process of claim 6, wherein the acidicsolution used in step 1) is selected from the group consisting ofhydrochloric acid, nitric acid, sulfuric acid, and phosphoric acid. 8.The process of claim 7, wherein the acidic solution has a concentrationof 1-10 mol/L.
 9. The process of claim 8, wherein, in step 1), the ratioof the weight of A₂Nb₄O₁₁ to the volume of the acidic solution is from 1g:10 ml to 1 g:600 ml, and the duration of the immersing is 10-96 hours.10. The process of claim 6, wherein the nitrogen source is an ammoniumsalt or a nitrogen-containing organic compound.
 11. The process of claim10, wherein the nitrogen source is urea or ammonium carbonate.
 12. Theprocess of claim 6, wherein, in step 2), the weight ratio of A₂Nb₄O₁₁and the nitrogen source is from 1:0.5 to 1:10.
 13. The process of claim6, wherein, in step 2), the heating is performed under a temperature of300-600° C., and the duration of the heating is 1-10 hours.
 14. Theprocess of claim 6, wherein, in step 2), nitrogen doped A₂Nb₄O₁₁ iswashed with acetone and/or distilled water, and dried under atemperature of 20-300° C. for 10-96 hours.
 15. The process of claim 14,wherein nitrogen doped A₂Nb₄O₁₁ is washed with distilled water, anddried under a temperature of 20-300° C. for more than 10 hours.
 16. Amethod for degradation of organic pollutants, comprising contacting theorganic pollutants with the compound of claim 1.